Molecular imaging by MRI comprises "smart labeling" with paramagnetic contrast agents and high magnetic field imaging for high spatial resolution. It provides a special tool for molecular biologists to visualize molecular events in vivo non-invasively thus to detect molecular and cellular mechanism of diseases. For diagnosis of diseases in the early stage, where only a few cells behave abnormally, the MRI contrast of paramagnetic labeling becomes critically important. Though 3 Tesla scanners are used in clinics to increase MR imaging sensitivity, the relaxivity of contrast agents decreases significantly at the magnetic field strength greater than 0.5 Tesla thus may reduce the imaging contrast for the paramagnetic labeling. Currently, there is no solution to this problem except using much higher dosage of contrast agents to compensate the relaxivity loss. Nevertheless, not all physiological conditions in vivo can allow or tolerate high concentration of contrast agents. To overcome the relaxivity loss without the use of high dosage of contrast agents, we have developed a unique methodology, the paramagnetic enhancement in off-resonance rotating frame, to modify the field dependence of contrast agents. Our preliminary studies have demonstrated that the methodology is feasible to differentiate tissues with very low concentration of contrast agents. Thus there is great potential to increase the MRI contrast at low dosage of contrast agent and high magnetic field. The goal of this proposal aims at advancing the understanding of the paramagnetic enhancement in off-resonance rotating frame on phantoms and examining the paramagnetic enhancement in off-resonance-rotating frame MR image in vivo. Several factors will challenge the paramagnetic enhancement in vivo, including the molecular structure of contrast agents, dynamic environment of contrast agents, and water diffusion exchange between various compartments. In order to establish the basic enhancement dependence on these factors, we propose to examine the residual magnetization dispersion profiles in off-resonance rotating frame for well-defined phantoms containing gadolinium-based macromolecular contrast medium, i.e., a macromolecule with covalently attached gadolinium chelates. Further, we will examine the paramagnetic enhancement in rat brain at high magnetic field following injection of the macromolecular contrast medium at reduced dosage. We will evaluate the paramagnetic relaxation enhancement according to the characteristics of residual magnetization dispersion profile for each voxel. We will generalize a method for assessing the quantity and environment of a paramagnetic probe required in specific application such as imaging drug delivery, cell migration and tumor growth.